The rotor of an electrical machine typically comprises a rotor core secured to a shaft. The rotor core may comprise a magnet having a bore through which the shaft is received. Most magnets are relatively brittle and will fracture if subjected to excessive tensile stress. Consequently, rather than press fitting the magnet on to the shaft, the magnet is generally adhered to the shaft.
The magnet may be adhered to the shaft by applying a bead of adhesive to the shaft and then inserting the shaft into the bore of the magnet. During insertion, the shaft may be rotated relative to the magnet so as to achieve a better distribution of adhesive. Nevertheless, as the length of the magnet increases, it becomes increasingly difficult to achieve a continuous distribution of adhesive along the full length of the bore. This in turn results in a weaker join between the magnet and the shaft.
The magnet may alternatively be adhered to the shaft by inserting the shaft into the bore of the magnet and then injecting adhesive into the clearance between the shaft and the magnet. However, it is generally difficult to deliver adhesive along the full length of the bore without trapping pockets of air. This is particularly true when the clearance is relatively small. The net result is a weaker join between the magnet and the shaft.